1. Field of the Invention
The present invention relates to a work vehicle that includes a pair of electric motors independently running and driving right and left running wheels in a state of being able to cause a straight running state and a turning running state; a pair of right and left change speed operation tools independently manually operated and performing a change speed operation to the right and left running wheels; a pair of change speed operation position detectors detecting an operation position of the respective change speed operation tools; and a controller controlling operation of the respective electric motors by setting target speeds with regard to the respective electric motors based on detection information of the respective change speed operation position detectors.
2. Description of Related Art
Conventionally, in the work vehicle having such a structure, when a change speed operation is performed to the right and left running wheels by the pair of right and left change speed operation tools, the operation of the respective electric motors is controlled by setting a target speed obtained from the operation position of the change speed operation tools as is (see Related Art 1).    [Related Art 1] Japanese Patent Laid-Open Publication No. 2008-168870
By performing a change speed operation to the right and left running wheels with the pair of right and left change speed operation tools, the work vehicle having such a structure can perform turning running in the forward and backward directions including turning running with a small turn such as a spin turn by driving the right and left running wheels in the reverse direction with respect to each other as well as running forward and running backward. The work vehicle can thus be preferably used for a mower and the like.
In such a structure, however, the operation of the respective electric motors is controlled by setting a target speed obtained from the operation position of the pair of right and left change speed operation tools as is. Consequently, there are disadvantages as follows:
For example, in a case of running straight in a forward direction, a driver manually operates the pair of right and left change speed operation tools for the same amount in the same direction for straight running so as to drive the right and left electric motors at the same speed in the forward direction. When the driver operates the change speed operation tools separately by the right and left hands, however, it is difficult to operate the operation positions on the right and left sides in the same position. It also happens that the change speed operation tools is swayed by shaking and the like of the vehicle body while running the work vehicle. There is thus a strong likelihood that the target speeds of the right and left electric motors will be slightly different.
In a case of running in a state where the target speeds of the right and left electric motors are different, the electric motor corresponding to the running wheel on the high-speed side tries to drive the running wheel by outputting driving force for straight running to achieve the target speed in accordance with the operation position commanded by the corresponding change speed operation tool. On the other hand, since the speed of the running wheel on the low-speed side is lower than the speed of the running wheel on the high-speed side, the running wheel on the low-speed side rotates under the influence of the driving force of the running wheel on the high-speed side, and thereby tries to rotate at a higher speed than the target speed in accordance with the operation position commanded by the change speed operation tool.
In such a case, the electric motor that drives the running wheel on the low-speed side generates torque of a reverse direction to control and adjust the speed to be its own target speed. On the other hand, since the running wheel on the high-speed side rotates under the influence of the driving force of the running wheel on the low-speed side, and thereby tries to rotate at a lower speed than the target speed in accordance with the operation position commanded by the change speed operation tool, the electric motor that drives the running wheel on the high-speed side generates torque to accelerate and adjust the speed to be its own target speed.
As a result, the difference in the actual running speed between the right and left running wheels becomes greater than the difference in the target speed in accordance with the operation positions commanded by the corresponding change speed operation tools, and the vehicle body of the work vehicle becomes more inclined on the either side of right and left from the straight moving state. The driver then tries to operate the change speed operation tool of the running wheel on the low-speed side toward the high-speed side and operate the change speed operation tool of the running wheel on the high-speed side toward the low-speed side so as to correct the balance. However, the running wheel on the low-speed side and the running wheel on the high-speed side become opposite with regard to right and left, and a similar state of rotating under the influence of the other occurs. Consequently, the difference in the speed between the right and left running wheels becomes greater than the difference in the target speed in accordance with the operation position commanded by the corresponding change speed operation tools, and the vehicle body of the work vehicle becomes more inclined on the either side of right and left from the straight moving state.
Specifically, a state of rotating under the influence of the other occurs repeatedly while reversing right and left, which results in hunting and deterioration in the running stability.